How can I model something that behaves like molasses?
For a still image, many tools can be used: an experienced 3D artist will probably use just mesh editing or sculpting. Someone would use particles and meta objects. I would personally go with a two step approach: fluid simulation first to get an overall realistic shape, then sculpting, mostly to smooth it well and make it look nice.
TL;DR
The very short answer if you're going down the &quot;Fluid simulation&quot; route is: a low &quot;Exponent&quot; (1 - 2) in the Fluid &gt; Domain &gt; Viscosity settings makes a thick fluid.

Step by step example

Scale. Make sure your existing objects (empty candy and plane) have the dimensions of a few decimeters = ~0.1 to 0.5 Blender units in v2.79b (it's not important that they are exactly real-size, EDIT: and actually if they are too small the fluid simulation could not work at all, but they shouldn't be 1 unit long either, i.e. 1 meter). Resize them, then apply their scale (CtrlA).

Set up a Fluid simulation. Like this:

a Fluid-type ball inside the cavity
your existing candy as Obstacle
a non-slip Domain cube with its bottom face corresponding to your standing surface. Reduce Exponent in viscosity to 1 or 2
set the Final resolution to a medium value (e.g. 120) and the Viewport Display to Final, and reduce the Scene's End time to something like 80. You don't need 250 frames for this!



Bake the fluid cache. Then choose a frame that you like, go to the Modifier tab and Apply the Fluidism modifier


Sculpt. I would mostly use the Smooth brush, and a bit of Inflate and Grab to add variation


Shading. Give it a specular, smooth, transparent, translucent material.

(I've also added some volume-emitted particles as bubbles, with a transparent, IOR&lt;1 material)